There are a few dream collectibles for me.
Possible (But Probably Not Happening)
Yule Log: Been kicking myself for a while about not opting for the Yule Log when it was available for purchase. Especially because of a lineup I've been contemplating for a little while. There aren't very many of them, and I'm not, never have been, and likely never will be particularly bell rich, so I doubt I'll find one.
の: It'd work really well for a particular lineup of mine, but they're way too pricey so I'll probably never own one.
Black Feather: I'd kind of like one because of a reference to my favorite video game, but so pricey I'll probably never bother. Not sure what sort of lineup to make even if I got one though.
White Feather: Same reasoning as Black Feather.
Celeste Chick Plush: Birds are my favorite type of creatures, owls are neat, Celeste is cute, and this plush is adorable. Too expensive though. Theoretically could have owned one, but was thwarted by limited restocks, so I sold star fragments and bought myself a Poké Ball collectible from someone instead. Worked itself out in the end since I still got something out of that Fair, but I technically missed my real, tangible chance at a Celeste plush and I doubt I'll ever find myself in the market for one.
The Stars Must Align, RNGesus Must Smile Upon Me, and I'd Probably Have Bad Luck To Make Up For It Afterward
Pink Candy: Oh my god, you guys, I almost could have had this last Halloween but I second guessed myself with the candy jar counting and chose a different number. My original guess, if I stuck with it, would have been 1 off from the actual number and I would've earned a Pink Candy. I have to assume that is the closest I will ever have been to earning a Pink Candy and I squandered it.
Dark Candy: Even more difficult to acquire than a Pink Candy? Yeah, no, not happening. Would be cool though.
Golden Easter Egg: I've never successfully solved every single egg hunt clue on these forums and probably never will. I'd absolutely love it for my egg lineup though.
Impossible
Pumpkin Pie: I cannot express to you how deeply disappointed I am that I failed to earn a pumpkin pie collectible. I mean, look at my name. I love Autumn, Halloween, pumpkin foods. It's my thing. Alas, it is not, in fact, my thing.
Ancient Lantern: Fits my Halloween aesthetic and just looks cool with that purplish light. It was from before my time though. Even if it does pop back up eventually, I doubt I'll earn one.
Ancient Scroll: Not as enthused about it as the Ancient Lantern, but still cool and Halloweeny. Didn't manage to snag one in last year's Halloween festivities though, and it was again otherwise before my time
Tetris Grid: Have an idea for what I'd do with my avatar and signature if I had one.
Shooting Star: Not much to say, really. It just looks really cool. I have lineup ideas for it.
(Green) Pinwheel: Another case of I know what I'd do with it if I had one in terms of lineup and avatar/signature. Would alternatively be fine with the regular Pinwheel, but yeah, that's too expensive and not happening either, so they'll share a slot here.
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If I had to pick just one over all the others that I'd want, I'm thinking Pumpkin Pie.